Balanced modulator circuit



c. E. ATKINS 2,441,127

BALANCED MODULATOR CIRCUIT May 11, 1948.

2 Sheets-Sheet 1 Filed Sept. 10, 1942 0U TPUT INVENTOR CA RL E. A TKl/VS ATTORNEY May H, 1948. c, Ns 2,441,127

BALANCED MODULATOR CIRCUIT Filed Sept. 10, 1942 2 Sheets-Sheet 2 INVENTOR CARL E. A TK/NS BY W1 l ATTORNEYS fiatenteci Ma 11 UNITED s TAT-ES PATENT "OFFICE 2341,12? meme) nenmfieeeieeen Edward Atkins, Evans'to'n, 111., ti'ssigner to Tung-Sol Lamp W'orks, InozyNewark, N. J1, a

"'c'orporatidnbf Delaware htiiiieetmnsepteifitr 10, mire-seer fie. istitiit teams. (01. 179 1715) 1 My present invention relates to balanced motlulator circuits and comprisesan improved circuit of this type which will insure substantially complete'elimination of the beating inputfrequen cies and of theharmonic'sjtherdf, passing only the and difierence frequencies. The new circuit may-be advantageously employedfor transmission of sum and difference'freqliericis'of"alhy twofin put frequencies, but is ofparticular valuefor juse in radio "systems wherein on'e or theother' of the beating input frequencies is so nearly the sarnejas the desired sum or difierence frequencies er the output that theseiective' means in *theoutpiit cireiiit of the usualbalancedmodmatoris inefiective to remove the input frequency. 'Thenew eireuit requires but two tubes, the anodes, screen "grids fa'nd cath0des ofwhi-ch areparallel connected. The 'new circuit includes also adjust'able means for suppressing'harmonie distortion. V e x The inventionis illustrated 'in the accompanying drawing, of which v H e Fig. 1 is acircuit diagramfofabalaneedmodulatdr circuit embodying the *mvention and 'inqluding twoinput transformers each cbnneeted in push' pull with control grids of a pair 'oftubes;

Fig. 2 is a circuit'dra'wing er a bem eedmedul'ator circuit similar to thatof Fig. l but indluding oh'e pu's'h-pull and one single ended input transformerjan'd w e v Fig. 3' is (a circuit diagramof a'balanced modulat'or 'illustrating an embodim'e'rit "of the invention *'requi r1ngno push-pu1l transformers, H W

The 'circuit of "Fig.1 -ineludes two mum-element tubes 2 and "4, which may be, for xempi'e, otthetypeknown as BSA'land twoi'n'pu't trans, formers *6 and '8. The anodesjlq of the tubes 2 and 4- are connected in parallel through a suitable load impedance 12 to a suitable so reedr p s t v' potential and the spreengrids 14 are "suitably polarized in the usual manner. The cathodes-f6 of the tubes arecon'ncted'in pairalleland gree eed through an adjustable resistor l'8. Trans"- fornier 6 feeds the outer cqntrol grids -20 of the tubesfin'push-pull'and the transformer '8 similarly feeds the inner control grids 22 er the tubes I With the above described circuit, 'wheneither p'airof grids 20 or 22 is independently energized whilefthejother fiai r' of grids isfguiesdent, the tunes will supply'equa'l "and opposite currentsto the load impedance 1'2 and hence the net output voltages fz'ejro. If, howeven bdth -pairs rgr1es-are s imul nfie s n re z .1 r d we e ri fl i e ne f61z eiy ie h t The purpose or the variahl'eresistanee 1-8 in the 12 oiricifit hf the eathdeis "I6 is to suppress in "the output "circuit "the eve 'harmenies, per'ti' men the second hermetic, of tn rrequency applied to tii in gr' szz, Thebanner-monieserthempi-it' frlequli'cyfibfn trahsformer' 8 are'cancled by tnepujsh-iiuu ebnn'ection whih' else b'alanees out the fundamental hut, w th th'edeseribd circuit, ifft-he inner rids'22 fare vigorously driven a possimy trouhlesome's'ecoridharmonic would appeafin the oiltputcirliit if ho rh''an'sitvere pro'vid'ed or it's sup ression. The impedance of resistance i8, w'nie'n it Will be hot'e'dji's not supplied with a bypassmg' eeneenser, serves to "degenerate tnisseeoiid'harmoni'c,;as well'as'a'ny other 'evenharmohic, 'sine the even harifionic currents are similar I'f'or the two tubes. There is no "harmonic degeneration for thesi'gnalPapphedto the outer control grids 20 and this 'nodoubt "accounts for the fact that the 'heterodyne frequencies are "not in the least affected By the presence of cathode to greund "impedance.

The circuit of 'FifiZ differs from that of Fig. "1 rrmerny in 'the 'dnne'ction to the inner control grids'zz of tubesz'and 4. Ihthis arrangeinent'the seondary of 'atransformer 'MsuppIying one input frequency is cdnnectedacross the control grid 22 and the eatho'de'of tubecZ while the inner control gridZTOf tubefd is'grounded. The cathode of "tube 2 is*grounded through a resistor '26, a variable part of which is in 'th'e'ground'circuit of thecathode of tube 4-. As in the circuit of Fig. 1, the outer control grids of tubes 2 and'4 are c'onnec'tedin'pushpull relation with the'second'ary or transformer 6, the an'odes T0 are 'eonnecte'd in parallel and supplied 'with positive potential through the load imf- 'pencian'ce lz and the scr'een grids l4 afeconne'cted to a suitable Source of positive potential.

Inthe above de'scrihd'oircuit, 'wnena signal is applied to the inner grid 22 of tube 2 an alterna'ting current "flows in its cathode circuit. thiseurrent is in phase "with the grid V'o1t'a'ge,'any potential ro across the cathode resistor w ll likewise have the same phase englefas the fgrid fvolt ag'e. Thappli'ceiti'on (if this Voltage 'erop to the 'athdde of'tuhe'q causes the current th'rdiih ttbje 4 to 'be out of phase 'With the cur ent through tube "zend aeeer'ainely the output currents of the two tubes een p medete cancel w en input voltages are applied onlyto the innercdnt'rol grids zg. 51 .i' efepp e 6 1 to w i e ffifi t q es 2 i hu e i i 1 e mn li t o i bode "urifent w re i lf lfie 'efi these gri .is reseiii tl. tron cur' s p on to th'e ano s tam-ea hac'k 'tb the sheen-ends 1'4; entire-refersistive impedance in the cathode ground circuit serves to cancel, while similarly large excursions on the outer control grids can be tolerated as but a negligible amount of harmonic distortion is produced. Also, as in the case of the circuit of Fig. 1, sum and diflerence frequencies are proance which is produced at the heterodyne rate.

4 output circuit. If the load impedance at is tuned, the selectivity of the output circuit adds to the attenuation of the input signals already provided by the balancing action of the modulator circuit,

In the circuit of Fig. 3 the grid leaks 14 and 16 have been shown and described as adjustable resistances rather than vpotentiometers, as it is important that there be no resistance between the second grid of one tube and the fourth grid of the other, that is, between grids 58 and 10 and between grids 66 and 66. These resistances 14 and 16 arepreferably of a low value, say, only a few thousand ohms in order to avoid the effects of plate-grid capacity and space-charge coupling,

which, if appreciable, would cause the phase angle of the cancellation voltages to depart from duced by virtue of the dlflerential transconduct-Y In many instances it is of decided advantage to dispense with push-pull transformers, and hence, in Fig. 3, I have shown a balanced modulator circuit embodying the invention in which both input signals may be provided by singleended sources. In this arrangement, as in that of Fig. 1, two multi-element tubes having their anodes, screen grids and cathodes parallel connected, are employed. In this instance the two tubes 28 and 30 are of the 6A8 variety. The anodes 34 are supplied with positive potential through a load impedance 36 which may be aperiodic or tuned. The parallel connected screen grids 38 are suitably polarized and by-passed to ground by a condenser 46. The parallel connected cathodes 42 are connected to ground through the biasing network 44. One input signal is impressed upon the inner grid 46 of tube 28, condenser 48 and grid leak 50 forming the coupling link to the signal source, and the other input signal is impressed upon the inner grid 52 of tube 30, condenser 54 and grid leak 56 forming the coupling link to this signal source. The anode side-rods or second grids 58 and 60 of tubes 26 and 30, respectively, are suitably polarized in series with respective load resistances 62 and 64. Grid" 66 of tube 28 is connected through blocking condenser 68 with grid 60 of tube 39 andgrid it! of tube 30 is similarly connected through a condenser 12 with grid 58 of tube 26. Adjustable grid leaks l4 and 16 provide D. C. return paths for grids 66 and 10, respectively, and also serve for adjusting the system.

In operation of the above described circuit, if energy is supplied from one source only, there will be no net voltage appearing across the loan impedance 3 6. Assume, for example, that, grid 52 of tube 36 is energized while grid 46 is quiescent. Current will then flow through tube 30 and a corresponding voltage appears across the load impedance 36. However, grid 66 of tube 30 also takes an alternating plate current of the same signal frequency, and hence an amplified voltage appears across resistor 64, which voltage is 180 degrees, out of phase with that on grid 52. Accordingly, this voltage, when applied to grid 66 of tube 28, causes an alternating space current of opposite direction to flow through tube 28, with the result that the voltage appearing across load 36, due to current through tube 30, may be balanced out by that through tube 28 by suitable ad- J'ustment of the grid leak 14. Similarly the voltage applied to grid 46 from the other source may be balanced out of the load circuit 36 by adjustment of grid leak 15. Sum and difference frequencies resulting from the interaction of the two input frequencies will, however, appear in the degrees sufficiently to impair the balance of the circuit unless neutralized by the employment of conventional means for the purpose. Tuned circuits could be substituted for the load resistors 62 and 64, if desired, but as resistors prove satisfactory and are simpler,-they are preferred.

As heretofore 1 indicated, the new balanced modulator circuit, because of the substantial suppression of both input frequencies from the output circuit, is of particular value when the heterodyne frequencies do not difier substantially from one or both of the input frequencies, as in this case the filtering out of one or the other of the input frequencies from the output circuit is not feasible.

I claim: 7

1. A balanced modulator circuit comprising a pair of multi-element tubes, each having an anode, a cathode'and at least two grids, an output circuit to which the anodes of said tubes are parallel connected, an impedance connected to said cathodes so as to have at least a part thereof in the common grid-cathode anode-cathode circuit of each tube, means for exciting one grid of each tube with alternating voltage of'one frequency and means for exciting another grid of each tube with an alternating'voltage of another frequency, each excited grid of one tube being phased oppositely to its companion grid in the other tube, said tubes each including a third grid and each of said means for exciting the first two mentioned grids of each tube including said third grid of one tube and a connection therefrom to one of the pair of grids of the other tube.-

2. A balanced modulator circuit-comprising in combination a pair of multi-element tubes each having an anode, a cathode, inner and outer control grids and a pair of screen grids connected together, one of said screen grids being disposed between the outer control grid and .the anode, and the other of said screen grids being disposed between the control grids, a load impedance, parallel connections from said anodes to said load impedance, an input transformer having its secondary connected in push-pull with said outer control grids to excite the same with energy of one frequency, a cathode circuit including a resistance to which said cathodes are connected so as to have at least a part thereof in the common grid-cathodeanode-cathode circuit of each tube, and means for exciting said inner control grids with energy of another frequency to produce alternating potentials of heterodyne frequencies across said load impedance, said resistance being grounded at one end, and being connected at its other endto the cathode of one tube and at an intermediate point to the cathode of the other tube and said. means for excitingfsaid inner grids comprising a' transformer having its secondary connected across the inner grid and the cathode of one tube, the inner grid of the other tube being grounded whereby the voltage drop across said resistance due to current through said first mentioned tube causes the currents through said tubes due to energy supplied by said last mentitgied transformer to be out of phase with each ot er.

3. A balanced modulator circuit comprising in combination a pair of multi-eiement tubes each having an anode, a cathode and at least three grids, a load impedance, parallel connections from said anodes to said load impedance, at common cathode circuit for said cathodes, means for exciting one grid of one tube with energy of one frequency, means for exciting the corresponding grid of the other tube with energy of another frequency, load circuits for the second grids of each tube, connections including a capacity from the second grid of each tube to the third grid of the other tube, and adjustable grid leaks for said third grids whereby current through one tube due to excitation of its first grid causes out of phase current to flow through the load circuit of its second grid and corresponding voltages to appear on the third grid of the other tube tending to suppress voltages across the load impedance of the frequency of the exciting energy.

4. A balanced modulator circuit comprising in combination two multi-element tubes, each having an anode, a screen grid, a cathode and two control grids, the anodes, screen grids and cathodes of said tubes being parallel connected, pushpull input connections to one control grid of each tube for exciting the same with energy of one frequency, push-pull input connections to the other control grid of each tube for exciting the same with energy of another frequency and a variable resistance connected between ground and said cathodes to cause degeneration of even harmonics of the input frequencies.

5. A balanced modulator circuit comprising in combination a multi-element tube having an anode, a cathode and inner and outer control grids, an input transformer having its secondary connected across said inner grid and said cathode, a resistance grounded at one end and connected to said cathode at the other end, a second multi-element tube having an anode, a cathode and inner and outer grids, the inner grid of said last mentioned tube being grounded and the oathode thereof being connected through a variable part of said resistance with the cathode of said first tube, a second input transformer having its secondary connected in push-pull with the outer grids of said tubes, a load impedance and parallel connections thereto from the anodes of said tubes.

6. A balanced modulator circuit comprising in combination a multi-element tube having an anode, a cathode and inner and outer control grids, an input transformer having its secondary connected across said inner grid and said cathode, a resistance grounded at one end and connected to said cathode at the other end, a second multi-element tube having an anode, a cathode and inner and outer grids, the inner grid of said last mentioned tube being grounded and the cathode thereof being connected through a variable part of said resistance with the cathode of said first tube, a second input transformer 6 having its secondary connected in push-pull with the outer grids of said tubes, a load impedance and parallel connections thereto from the anodes of said tubes, and a screen grid for each tube adapted to be parallel connected to a source of positive potential.

7. A balanced modulator circuit comprising a pair of multi-element tubes, each having an anode, a cathode, two control grids and a pair of screen grids connected together, one of said screen grids being disposed between a control grid and the anode, and the other of said screen grids being disposed between the control grids, an output circuit to which the anodes of said tubes are parallel connected, at least a part of said impedance being in the common grid-cathode anode-cathode circuit of each tube, a cathode circuit including a resistive impedance to which the cathodes of said tubes are connected, means for exciting one control grid of each tube with alternating voltage of one frequency and means for exciting the other control grid of each tube with an alternating voltage of another frequency, said resistive impedance serving for degeneration of the even harmonics of one of the exciting frequencies.

8. A balanced modulator circuit comprising a pair'of multi-element tubes each having a cathode, an anode and at least two control grids, means for exciting in opposite phase one grid of each tube with an alternating voltage of one frequency, means for exciting in opposite phase the other control grid of each tube with an alternating current of another frequency, a common load for said anodes and means common to said cathodes for degenerating the even harmonics of at least one of the exciting frequencies.

9. A balanced modulator circuit comprising a pair of multi-element tubes, each having an anode, a cathode and at least two grids, an output circuit to which the anodes of said tubes are parallel connected, means for exciting one grid of each tube with alternating voltage of one frequency and means for exciting another grid of each tube with an alternating voltage of another frequency, and a variable resistance connected to said cathodes so as to have at least a part thereof in the common grid-cathode anode-cathode circuit of each tube for degenerating the even harmonics of one of the exciting frequencies.

CARL EDWARD ATKINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,855,509 Blair Apr. 26, 1932 1,926,875 Llewellyn Sept. 12, 1933 1,977,536 Tubbs Oct. 16, 1934 2,031,639 Finch Feb. 25, 1936 2,171,151 Urtel Aug. 29, 1939 2,185,367 Blumlein Jan. 2, 1940 2,239,776 Brunn Apr. 29, 1941 FOREIGN PATENTS Number Country Date 485,642 Great Britain Aug. 18, 1936 Certificate of Correction Patent No. 2,441,127. May 11, 1948.

CARL EDWARD ATKINS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Column 3, line 54, for the Word loan read load; column 6, lines l5, l6, and 17, claim 7, strike out the Words and comma at least a part of said impedance being in the common grid-cathode anodecathode circuit of each tube, and insert the same after the comma and before means in line 19, same claim; and that the said Letters Patent should be read with these cmgections therein that the same may conform to the record of the case in the Patent 0 co.

Signed and sealed this 13th day of July, A. D. 1948.

[REAL] THOMAS F. MURPHY,

Assistant Commz'ssioner of Patents. 

